This invention relates to semiconductor structures such as infrared sensors, and, more particularly, to improving their operation by removing impurities therein which result from device fabrication.
A common physical configuration for many types of microelectronic devices, such as sensors and electronic circuitry, is a thin-film device deposited upon a substrate. The thin film device is formed of at least one layer of a semiconductor material having properties useful for the particular application, deposited upon the substrate. For optical sensors, the semiconductor material converts incident light to an electrical current. For electronic circuitry, the semiconductor material serves as the base for fabrication of active elements such as transistors as well as passive elements.
The optical and electrical performance of semiconductor materials is highly dependent upon elements present in the material in relatively small amounts. Some such minor elements, generally called dopants, are intentionally added to alter the performance in favorable and controllable ways. Other such minor elements are unintentionally present as a result of the fabrication of the device. If the structure is grown from the melt, impurities in the molten material may be frozen into the solidified semiconductor material. If the structure is deposited by a technique such as vapor deposition, impurities from the atmosphere, the substrate, or the source material may be co-deposited. In any case, the impurities create defects which alter the performance of the thin-film semiconductor device in an uncontrollable manner by mechanisms such as recombination, trapping, and tunneling.
Care is taken to minimize the presence of impurities during fabrication of the structure. However, it is not possible to achieve, by current fabrication techniques, sufficiently low concentrations of impurities for some applications in as-fabricated structures. Methods have therefore been developed to extract impurities from contaminated structures, after they are prepared. In some cases the extraction of impurities is quite difficult. Mobile impurities such as lithium, sodium, potassium, and copper may be removed from CdZnTe or CdTe by contacting the structure to molten potassium cyanide at temperatures greater than 634.5xc2x0 C. This technique is both potentially hazardous to practice due to the presence of cyanide and also requires quite a high temperature that is not suitable for many other semiconductor materials. All-solid processes have been attempted, such as the deposition of a sacrificial layer on a semiconductor layered structure. For example, a sacrificial layer of polycrystalline CdTe may be deposited on the surface of a HgCdTe layer, and the composite structure heated to the range of 200-400xc2x0 C. The objective is that impurities diffuse from the HgCdTe layer into the CdTe layer, and the CdTe layer is later removed. This technique has not proved to be successful. In a separate approach to altering properties of semiconductor structures, wash melts are sometimes used as part of the deposition and growth process. The wash melts are used to clean and etch the surface of the substrate, but are not contacted to the substrate for a sufficiently long time to extract impurities.
There is a need for a technique by which mobile impurities may be removed from structures such as thin-film semiconductor devices, particularly those containing mercury, cadmium, zinc, and/or tellurium as principal constituents. The present invention fulfills this need, and further provides related benefits.
The present invention provides a method for extracting impurities from contaminated thin-film structures whose principal constituents include mercury, cadmium, zinc, and/or tellurium. Examples of such materials include HgCdTe, CdTe, CdZnTe, and HgCdZnTe, all of which are substrate materials or infrared-sensitive detector materials. (Hg refers to mercury, Cd refers to cadmium, Zn refers to zinc, and Te refers to tellurium.) Contamination of such materials by impurities such as lithium, sodium, potassium, and copper adversely affect their performance, and the levels of such impurities cannot be reduced sufficiently by common fabrication procedures. The present invention provides for the removal of such impurities efficiently and at relatively low cost. It is accomplished at a sufficiently low contacting temperature to be suitable for extraction of impurities from materials such as the mercury-containing compounds HgCdTe and HgCdZnTe. The method does not require complex equipment, is safe, and is easy to practice. Variations of the basic method ensure that the principal constituents are not depleted.
In accordance with the invention, impurities are extracted from a contaminated thin-film structure comprising a constituent element selected from the group consisting of mercury, cadmium, zinc, and tellurium, and mixtures thereof, and having impurities therein. The method includes preparing a sink medium comprising molten bismuth and contacting the contaminated structure to the sink medium for a period of time of at least about 10 minutes.
In another embodiment, the method includes the step of preparing a sink medium comprising molten bismuth and at least one added constituent element selected from the group consisting of mercury, cadmium, zinc, and tellurium, and mixtures thereof. The step of preparing includes the substeps of furnishing bismuth, furnishing the at least one added constituent element from a source other than the contaminated thin-film structure, and mixing the at least one added constituent element and the bismuth together. The contaminated thin-film structure and the sink medium are contacted together for a period of time. The xe2x80x9cconstituent elementsxe2x80x9d of the contaminated structure are the elements which are desirably in the structure, and are to be maintained without a change in chemical composition. The xe2x80x9cimpurity elementsxe2x80x9d those which contaminate the contaminated structure and are to be extracted. Their chemical concentration is to be reduced.
The step of contacting is preferably performed at a contacting temperature of from about 275xc2x0 C. to about 325xc2x0 C. The period of time is preferably from about 10 minutes to about 48 hours.
The step of furnishing a sink medium may include the step of providing a bath of the sink medium comprising molten bismuth and mercury, under an inert atmosphere having an atmospheric concentration of mercury therein.
In one preferred embodiment, a method for extracting impurities is operable with a thin-film contaminated device including a principal constituent selected from the group consisting of mercury, cadmium, zinc, and tellurium, and mixtures thereof, and having impurities therein. The contaminated device comprises a layer structure with a thickness of no more than about 200 micrometers, preferably from about 5 to about 50 micrometers, and most preferably about 20 micrometers. Some layer structures of particular interest are the layer structures comprising at least one layer selected from the group consisting of HgCdTe, CdTe, CdZnTe, and HgCdZnTe. The method includes furnishing a sink medium comprising molten bismuth, and contacting the contaminated device to the sink medium for a period of time. This contacting at elevated temperature causes mobile impurities such as lithium, sodium, potassium, and copper to diffuse through the thin-film device to the free surface, and to transfer to the molten sink medium, which thereby serves as a diffusion sink for the impurities.
In one embodiment, the layer structure comprises at least two constituent elements, and the step of furnishing includes the step of furnishing the sink medium comprising molten bismuth and at least one of the constituent elements of the layer structure. At least one of the two principal constituent elements may be selected from the group consisting of mercury cadmium, zinc, and tellurium. Desirably, the element with the highest vapor pressure would be present in the sink medium. If the layer structure comprises mercury, the sink medium may further comprise mercury; if the layer structure comprises cadmium, the sink medium may further comprise cadmium; if the layer structure comprises zinc, the sink medium may further comprise zinc; if the layer structure comprises tellurium, the sink medium may further comprise tellurium; if the layer structure comprises two or more of these principal constituent elements, the sink medium may comprise each of the principal constituent elements that is present. The higher the vapor pressure of the constituent element, the more preferable that it be included in the sink medium to reduce loss of the element from the layer structure.
The contacting of the contaminated thin-film device to the sink medium is preferably accomplished by providing a bath of the sink medium at the desired contacting temperature of no less than the melting point of the sink medium. The sink medium is primarily molten bismuth, but it may contain small amounts of one or more of the primary constituent elements. The bath is desirably covered by a chemically inert vapor of a gas such as argon, helium, or nitrogen, or a reducing gas such as hydrogen. One or more of the primary constituents of the layer structure may also be present in the vapor. The presence of the primary constituent elements in the sink medium and in the atmosphere increases their chemical activity so that there is less tendency for the diffusional loss of the primary constituents from the thin-film device into the sink medium. The thin-film device is immersed into the sink medium for a sufficient period of time to reduce the impurity content(s) to the desired level(s).
The present approach uses bismuth as the primary component of the sink medium. Bismuth has a relatively low melting point, so that the contacting temperature may be sufficiently low that the method is practical for use with mercury-containing layer structures and other structures where interdiffusion and/or loss of the principal constituents is a concern. The contacting temperature may be sufficiently high that the mobile impurities diffuse at acceptably high rates and the extraction may be accomplished in commercially practical times. Molten bismuth also dissolves mercury in acceptable amounts to provide the activity increase discussed above.
The present invention thereby achieves extraction of impurities from a contaminated thin-film structure without adversely affecting the thin-film structure itself. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.